Recently, in the field of display systems, displays that use various display devices, such as a liquid crystal display system and a display that uses organic EL (Electro Luminescence) elements, have been developed. Higher image quality (increased gray scales) is demanded for these display systems, and voltage amplitudes of a scan signal and a gray scale signal tend to be increased. For this reason, higher voltages of a row driver that drives a scan line of a display panel and a column driver that drives a data line of the display panel using the gray scale signal are demanded.
On the other hand, higher-speed transfer and low EMI (Electro-Magnetic Interference) using a small number of signal lines are demanded for various control signals and image data signals supplied from the row driver and the column driver from a display controller. Lower amplitudes of those signals are being achieved.
Further, even in the row driver and the column driver, a fine fabrication process has been adopted in order to reduce an increase in the area (cost) of logic circuits that handle data, the amount of which increases accompanying a higher definition and the increased gray scales. With a fine fabrication process, the power supply voltage of the logic circuit tends to be reduced.
That is, lower voltages of input sections of the row driver and the column driver and higher voltages of output sections of the row driver and the column driver are demanded.
For this reason, in a level shift circuit that converts a low-voltage signal in an input section thereof to a high-voltage signal in an output section thereof, a low-amplitude signal must be converted to a high-amplitude signal at high speed.
FIG. 21 is a diagram showing an example of a typical configuration of the level shift circuit that converts a low-amplitude signal to a high-amplitude signal (refer to Patent Document 1 listed below). Referring to FIG. 21, this level shift circuit receives a low-voltage signal IN and outputs a high-voltage output signal OUT and a high-voltage output signal OUTB which is a reverse phase signal of the signal OUT. The level shift circuit includes P-channel MOS transistors P1 and P2 which have sources connected to a power supply terminal VDD3, have gates connected to output terminals W2 and W1, respectively, and have drains connected to the output terminals W1 and W2, respectively. The P-channel MOS transistors P1 and P2 function as charging elements for the output terminals W1 and W2, respectively. The P-channel MOS transistors P1 and P2 respectively receive at gates thereof the high-amplitude output signal OUT output from the output terminal W2 and the output signal OUTB output from the output terminal W1. The maximum absolute value of a gate-to-source voltage VGS of each of the P-channel MOS transistors P1 and P2 is |VSS−VDD3|. N-channel MOS transistors N1 and N2 function as discharging elements for the output terminals W1 and W2, respectively. N-channel MOS transistors N1 and N2 have sources connected to a power supply terminal VSS and have drains connected to the output terminals W1 and W2, respectively. N-channel MOS transistors N1 and N2 respectively receive at gates thereof the low-voltage input signal IN and the inverted signal of the input signal IN (both being low-amplitude signals).
The maximum gate-to-source voltage of each of the discharging elements N1 and N2 is set to the amplitude of the input signal IN. Discharging capability of each of the discharging elements N1 and N2 is lower than charging capability of each of the charging elements P1 and P2 of which the maximum absolute value of the gate-to-source voltage VGS is |VSS−VDD3|. A drain current of each of the discharging elements N1 and N2 and the charging elements P1 and P2 is proportional to the square of [(gate-to-source voltage)−(threshold voltage)]. A drain current of each of the charging elements P1 and P2, whose gate-to-source voltage in an on state is set to a larger value, is larger than a drain current of each of the discharging elements N1 and N2.
Then, in order to increase discharging capability of respective discharging elements N1 and N2, the element size of the respective discharging elements N1 and N2 (W/L ratio; where W is a channel width and L is a channel length) needs to be sufficiently large.
By the way, the discharging capability of respective discharging elements N1 and N2 must be set to exceed the charging capability of the respective charging elements P1 and P2. This can be readily understood in view of a discharging operation of the respective discharging elements N1 and N2.
As a specific example, a change from a state (initial state) where the output terminals W1 and W2 respectively have a potential VDD3, (which is a High potential) and a potential VSS, (which is a Low potential) will be considered, for example. In this state, the charging element P1 is in an on state, while the charging element P2 is in an off state. Further, the input signal IN is Low, the discharging element N1 is in an off state, and the discharging element N2 is in an on state.
Then, when the input signal IN is changed from Low to High, the discharging element N1 is turned on, and the discharging element N2 is turned off. However, the charging element P1 immediately after the change of the input signal IN from Low to High is kept in an on state. Thus, in order to change the output terminal W1 to Low (VSS) by the discharging element N1, the discharging capacity of the discharging element N1 (drain current of the NMOS transistor N1) needs to exceed the charging capability of the charging element P1 (drain current of the PMOS transistor P1).
Accordingly, in order to cause the level shift circuit in FIG. 21 to operate normally, the element size (W/L ratio) of the respective discharging elements N1 and N2 must be set to be sufficiently large, and also the element size (W/L ratio) of the respective charging elements P1 and P2 must be set to be sufficiently small so that the discharging capability exceeds the charging capability. That is, the size of each of discharging elements in the level shift circuit in FIG. 21 is increased and the area of the level shift circuit is increased. Especially when operated at low voltage of the input signal IN, the discharging capability of the discharging elements N1 and N2 is relatively reduced. Thus, the circuit area will further increase.
Further, it becomes difficult to set a transistor size so that the discharging capability of the respective devices N1 and N2 sufficiently exceeds the charging capability of the respective charging elements P1 and P2.
When W/L ratios of the discharging elements N1 and N2 are increased, a level shift operation is slowed down due to an increase in parasitic capacitances. Thus, a period where the discharging element N1 and the charging element P1 are simultaneously in an on state or a period where the discharging element N2 and the charging element P2 are simultaneously in an on state is prolonged. Thus, there also arises a problem that a short circuit current that flows transiently increases, so that power dissipation increases.
FIG. 22 is a diagram showing a configuration shown in Patent Document 1. Referring to FIG. 22, the level shift circuit in Patent Document 1 includes P-channel MOS transistors P51 and P52, P-channel MOS transistors P3 and P4, P-channel MOS transistors P1 and P2, and N-channel MOS transistors N1 and N2. The P-channel MOS transistors P51 and P52 have sources connected to a power supply terminal VDD3 and have drains connected to nodes W3 and W4, respectively. The P-channel MOS transistors P3 and P4 have sources connected to the power supply terminal VDD3 and drains connected to the nodes W3 and W4, respectively. The P-channel MOS transistors P1 and P2 have sources connected to the nodes W3 and W4, respectively, and drains connected to nodes W1 and W2, respectively, and gates cross-connected to the nodes W2 and W1, respectively. The N-channel MOS transistors N1 and N2 have sources connected to a power supply terminal VSS, drains connected to the nodes W1 and W2, respectively, and gates connected to an input terminal IN and an output of an inverter INV0 that inverts a signal from the input terminal IN. To gates of the P-channel MOS transistors P3 and P52, a signal obtained by inverting a signal at the node W2 by the inverter INV1 is supplied. To gates of the P-channel MOS transistors P4 and P51, a signal obtained by inverting an output of the inverter INV1 by an inverter INV2 is supplied. An output of the inverter INV2 is connected to an output terminal OUT. When the P-channel MOS transistors P51 and P52 are set to high-resistance transistors, and when one of control terminals of the discharging elements N1 and N2 is changed from a Low level of a low amplitude to a High level of the low amplitude, the level shift circuit in Patent Document 1 aims to readily reduce a voltage at the output node (W1 or W2), thereby achieving a high-speed level shift operation and suppressing a short circuit current.
As an initial state, for example, consider a state where the input signal IN is at a Low level of a low amplitude, the discharging elements N1 and N2 are turned off and on, respectively, the charging elements P1 and P2 are turned on and off, respectively, the nodes W1 and W2 are respectively at a High level and a Low level of a high amplitude, the P-channel MOS transistors P3 and P52 that receive the inverted-phase signal of the output signal of the output node W2 via the inverter INV1 are turned off, and the P-channel MOS transistors P4 and P51 that receive the in-phase signal of the output signal of the output node W2 via the inverter INV2 are turned on. In this case, the output node W1 is kept at the High level of the high amplitude by weak charging capability of the P-channel MOS transistor P51 that constitutes a high-resistance transistor.
Next, a time when the input signal IN is changed from Low to High from the initial state will be considered. At this time point, the discharging elements N1 and N2 are turned on and off, respectively, and the output node W1 is discharged from the High level to the Low level of the high amplitude by the discharging element N1.
Discharging capability of the discharging element N1 immediately after the discharging element N1 has been turned on must exceed charging capability of the charging element P1. In this case, the charging capability of the charging element P1 is determined by the high-resistance transistor P51. Thus, the discharging element N1 may change the output node W1 to a Low level comparatively readily.
The charging element P2 is turned on by the change of the output node W1 to a Low level. The output node W2 is thereby changed to a High level. In this case, charging capability of the charging element P2 is determined by charging capability of the P-channel MOS transistor P4. The charging element P2 may readily change the output node W2 to a High level.
When the output node W2 goes to a High level, the P-channel MOS transistors P3 and P52 of which gates are connected to the output of the inverter INV1 are both turned on, and the P-channel MOS transistors P4 and P51 of which the gates are connected to the output of the inverter INV2 are both turned off. With this arrangement, the output node W2 that has been changed to a High level of the high amplitude is kept at the High level by weak charging capability of the P-channel MOS transistor P52 that constitutes the high-resistance transistor.    [Patent Document 1] JP Patent Kokai Publication No. JP-P-2001-298356A